Download Chapter 3 - William Stallings, Data and Computer Communications

Data and Computer
Communications
Chapter 3 – Data Transmission
Eighth Edition
by William Stallings
Lecture slides by Lawrie Brown
Data Transmission
 Toto,
I've got a feeling we're not in Kansas
anymore. Judy Garland in The Wizard of
Oz
Transmission Terminology
 data
transmission occurs between a
transmitter & receiver via some medium
 guided medium

eg. twisted pair, coaxial cable, optical fiber
 unguided

/ wireless medium
eg. air, water, vacuum
Transmission Terminology
 direct

link
no intermediate devices
 point-to-point


direct link
only 2 devices share link
 multi-point

more than two devices share the link
Transmission Terminology
 simplex

one direction
• eg. television
 half

duplex
either direction, but only one way at a time
• eg. police radio
 full

duplex
both directions at the same time
• eg. telephone
Frequency, Spectrum and
Bandwidth
 time

domain concepts
analog signal
• various in a smooth way over time

digital signal
• maintains a constant level then changes to another
constant level

periodic signal
• pattern repeated over time

aperiodic signal
• pattern not repeated over time
Analogue & Digital Signals
Periodic
Signals
Sine Wave

peak amplitude (A)



frequency (f)





maximum strength of signal
volts
rate of change of signal
Hertz (Hz) or cycles per second
period = time for one repetition (T)
T = 1/f
phase ()

relative position in time
Varying Sine Waves
s(t) = A sin(2ft +)
Wavelength ()
 is
distance occupied by one cycle
 between two points of corresponding
phase in two consecutive cycles
 assuming signal velocity v have  = vT
 or equivalently f = v
 especially when v=c

c = 3*108 ms-1 (speed of light in free space)
Frequency Domain Concepts
 signal
are made up of many frequencies
 components are sine waves
 Fourier analysis can shown that any signal
is made up of component sine waves
 can plot frequency domain functions
Addition of
Frequency
Components
(T=1/f)
c

is sum of f & 3f




Frequency
Domain
Representations




freq domain func of
Fig 3.4c
 freq domain func of
single square pulse
< <
Spectrum & Bandwidth

spectrum


absolute bandwidth


width of spectrum
effective bandwidth
 often just bandwidth


range of frequencies contained in signal
narrow band of frequencies containing most energy
DC Component

component of zero frequency
Data Rate and Bandwidth

any transmission system has a limited band of
frequencies

this limits the data rate that can be carried

square have infinite components and hence
bandwidth

but most energy in first few components

limited bandwidth increases distortion

have a direct relationship between data rate &
bandwidth
Figure 3.7 (a) & (b)
Bit time = T / 2
Data Rate Calculation

Case 1




Case 2




Bandwidth 4MHz, use the sine wave of Fig. 3-7 (a)
4MHz = 5f – f  f = 1MHz
Data rate = 2 Mbps
Bandwidth 8MHz, use the sine wave of Fig. 3-7 (a)
8MHz = 5f – f  f = 2MHz
Data rate = 4 Mbps
Case 3



Bandwidth 4MHz, use the sine wave of Fig. 3-4 (c)
4MHz = 3f – f  f = 2MHz
Data rate = 4 Mbps
Data Rate vs. Bandwidth
 Bandwidth


Data rate ↑ (compare case 1 & 2)
Same signal quality
 Same



bandwidth
Higher signal quality  lower data rate
Compare case 1 & 3
 Same

↑
data rate
Bandwidth ↑  better signal quality
Compare case 2 & 3
Analog and Digital Data
Transmission
 data

entities that convey meaning
 signals

& signaling
electric or electromagnetic representations of
data, physically propagates along medium
 transmission

communication of data by propagation and
processing of signals
Acoustic Spectrum (Analog)
Audio Signals

freq range 20Hz-20kHz (speech 100Hz-7kHz)
 easily converted into electromagnetic signals
 varying volume converted to varying voltage
 can limit frequency range for voice channel to
300-3400Hz
Video Signals

USA - 483 lines per frame, at frames per sec


525 lines x 30 scans = 15750 lines per sec



have 525 lines but 42 lost during vertical retrace
63.5s per line
11s for retrace, so 52.5 s per video line
max frequency if line alternates black and white
 horizontal resolution is about 450 lines giving
225 cycles of wave in 52.5 s
 max frequency of 4.2MHz
Digital Data
 as
generated by computers etc.
 has two dc components
 bandwidth depends on data rate
Analog Signals
Digital Signals
Advantages & Disadvantages
of Digital Signals
 cheaper
 less
susceptible to noise
 but greater attenuation
 digital now preferred choice
Transmission Impairments
 signal
received may differ from signal
transmitted causing:


analog - degradation of signal quality
digital - bit errors
 most



significant impairments are
attenuation and attenuation distortion
delay distortion
noise
Attenuation

where signal strength falls off with distance
 depends on medium
 received signal strength must be:


strong enough to be detected
sufficiently higher than noise to receive without error

so increase strength using amplifiers/repeaters
 is also an increasing function of frequency
 so equalize attenuation across band of
frequencies used

eg. using loading coils or amplifiers
Delay Distortion
 only
occurs in guided media
 propagation velocity varies with frequency
 hence various frequency components
arrive at different times
 particularly critical for digital data
 since parts of one bit spill over into others
 causing intersymbol interference
Noise
 additional
signals inserted between
transmitter and receiver
 thermal



due to thermal agitation of electrons
uniformly distributed
white noise
 intermodulation

signals that are the sum and difference of
original frequencies sharing a medium
Noise
 crosstalk

a signal from one line is picked up by another
 impulse

irregular pulses or spikes
• eg. external electromagnetic interference




short duration
high amplitude
a minor annoyance for analog signals
but a major source of error in digital data
• a noise spike could corrupt many bits
Channel Capacity
 max
possible data rate on comms channel
 is a function of




data rate - in bits per second
bandwidth - in cycles per second or Hertz
noise - on comms link
error rate - of corrupted bits
 limitations
due to physical properties
 want most efficient use of capacity
Nyquist Bandwidth

consider noise free channels
 if rate of signal transmission is 2B then can carry
signal with frequencies no greater than B

ie. given bandwidth B, highest signal rate is 2B

for binary signals, 2B bps needs bandwidth B Hz
 can increase rate by using M signal levels
 Nyquist Formula is: C = 2B log2M
 so increase rate by increasing signals


at cost of receiver complexity
limited by noise & other impairments
Shannon Capacity Formula

consider relation of data rate, noise & error rate


faster data rate shortens each bit so bursts of noise
affects more bits
given noise level, higher rates means higher errors

Shannon developed formula relating these to
signal to noise ratio (in decibels)
 SNRdb=10 log10 (signal/noise)
 Capacity C=B log2(1+SNR)


theoretical maximum capacity
get lower in practise
Summary
 looked
at data transmission issues
 frequency, spectrum & bandwidth
 analog vs digital signals
 transmission impairments